1. Field of the Invention
The present invention is generally related to moisture-scavenging compositions, and more particularly, the invention is related to moisture-scavenging oxazolidines.
2. Background of the Art
Moisture-curable polyurethane coatings are extensively used as commercial and industrial protective and/or decorative coatings. Polyurethane coatings, known in the industry as one of the toughest coatings available, are routinely applied as protective coatings on interior and exterior walls of buildings, industrial machinery, military equipment and vehicles, commercial and passenger vehicles, and any other surface requiring a protective coating. Moisture-curable polyurethane systems are also used extensively as sealants and adhesives.
Moisture-curing polyurethane coating systems include a polyisocyanate component which reacts with atmospheric water at room temperature to form useful films. These systems also include pigments, organic solvents, and a variety of adjuvant components, e.g., surface active agents, dispersants, diluents, and fillers. This type of coating is one of the finest coatings available that can be produced without the application of heat or other external sources of energy. These systems are very useful for objects that cannot be heat-cured such as buildings, large machinery, airplanes, ships and vehicles.
Since the polyisocyanate component reacts with even trace amounts of moisture, extreme care must be taken so that the polyisocyanates do not contact water until they are applied to a surface to be coated. Water is, however, unintentionally and unavoidably introduced into the formulation process in the form of dissolved water in solvents, adsorbed and absorbed water in fillers and pigments, and atmospheric moisture. Subsequent reaction of the water with the polyisocyanate component of the system results in an irreversible reaction which will harden the product, making it unusable before it can be applied to the surface to be coated.
Moisture-curing polyurethane coatings are supplied as both one and two-part systems. In a two-part system, a polyol is chosen that will chemically react with and polymerize the polyisocyanate. The polyol portion generally includes a pigment. The pigmented polyol portion and the polyisocyanate portion are supplied in separate containers to the ultimate user or consumer. The consumer mixes the two portions in accordance with the ratio specified by the manufacturer just prior to use. This admixture has a limited life and must be used within a relatively short period of time or it will polymerize and become unusable in its container.
The two-part system presents several problems to the ultimate user. For example, there is a risk that the pigmented polyol portion and the polyisocyanate portion will not be mixed in the correct ratio. This could seriously detract from the performance of the resulting moisture-cured polyurethane coating. In addition, this approach is expensive in that material is wasted if the entire admixture is not used promptly.
Moisture-curing polyurethane coatings are also supplied as a one-part system eliminating all of the above mentioned problems of the two-part system. In the one-part system, however, extreme care must be taken to assure that all of the moisture is removed from the system prior to packaging the product; otherwise, the polyisocyanate component in the product will react with the available water and polymerize in the container. Sources of water in one-part systems include color pigments and fillers which contain adsorbed moisture on their surfaces, water dissolved in solvents, and atmospheric moisture. This water must be removed in order to produce an acceptable product. The existing methods for preparing color-pigmented moisture-curable polyurethane coatings in a single package require expensive equipment to dry the pigments, solvents, and fillers. In the alternative, moisture-scavenging agents are added to the preparation.
One group of moisture-scavenging compounds are the molecular sieves. Molecular sieves adsorb water into their pores, thereby binding the water and preventing it from reacting with the polyisocyanate component. An example of a molecular sieve is sodium potassium aluminosilicate, available from the Mobay Corp., Pittsburgh, Pa., under the tradename designation Baylith L Powder. One disadvantage of using molecular sieves is that they reduce the gloss of the cured coating. Another disadvantage of molecular sieves is that they will sometimes plasticize or embrittle the cured coating.
A second group of water-scavenging agents widely used to prevent moisture-contamination of moisture-curable polyurethane coating systems is the monomeric isocyanates. The monomeric isocyanates, such as p-toluenesulfonyl isocyanate (Vanchem, Inc. Lockport, Conn.), react with water to generate carbon dioxide and the corresponding sulfonamide, e.g., p-toluenesulfonamide. The carbon dioxide diffuses from the curing coating as carbon dioxide gas.
A disadvantage of monomeric isocyanates is that they are extremely corrosive to skin and toxic. In literature prepared by a supplier of monomeric isocyanates (Aldrich Chemical Co., Milwaukee, Wis.), monomeric isocyanates are referred to as harmful if swallowed, inhaled, or absorbed through the skin. Moreover, these compounds are listed as extremely corrosive to the tissues of the mucous membranes, upper respiratory tract, and skin. Symptoms of monomeric isocyanate exposure include burning, coughing, nausea and vomiting, chemically induced asthma, chest pain, and pulmonary edema possibly causing death. If these compounds are to be handled by the ultimate user, the manufacturer recommends a self-contained breathing apparatus, rubber boots and heavy rubber gloves. Since these compounds are extremely corrosive to skin and toxic, great care must be taken in their manufacture, handling, and use. This enormous health risk dramatically increases the cost of these compounds to the manufacturer and the ultimate user.
There is a need for a moisture-scavenger which efficiently, cost effectively, and safely removes water from moisture-curable polyurethane coating systems and from any other application where residual water is a problem, without seriously detracting from the performance of the cured coating.